I got a magnetic tank bag from Aldi a few years back and it is still serving me well. I don't seem to be able to master the art of travelling light so it gets nearly full just on a day trip with flask, camera, first aid kit, and other junk. You can increase capacity with a separate zip off section underneath the main section, but I found that makes it too high and floppy so I don't use that and rely on panniers/top box for extra capacity on longer trips. Aldi usually have the motorbike stuff early in the fine weather riders season.
Not a very good picture of the tank bag but at least the bikes clean.